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(EX/S1-5) Beta-limiting MHD Instabilities in NSTX Spherical Torus Plasmas

J.E. Menard1), M.G. Bell1), R.E. Bell1), E.D. Fredrickson1), D.A. Gates1), S.M. Kaye1), B.P. LeBlanc1), R. Maingi2), D. Mueller1), S.A. Sabbagh3), D. Stutman4), C.E. Bush2), D.W. Johnson1), R. Kaita1), H.W. Kugel1), R.J. Maqueda5), F. Paoletti3), S.F. Paul1), M. Ono1), Y.-K.M. Peng2), C.H. Skinner1), E.J. Synakowski1), NSTX Research Team
1) Princeton Plasma Physics Laboratory, Princeton, USA
2) Oak Ridge National Laboratory, Oak Ridge, TN, USA
3) Columbia University, New York, NY, USA
4) Johns Hopkins University, Baltimore, MD, USA
5) Los Alamos National Laboratory, Los Alamos, NM, USA

Abstract.  Global magnetohydrodynamic stability limits in the National Spherical Torus Experiment (NSTX) have increased significantly recently due to a combination of device and operational improvements. First, more routine H-mode operation with broadened pressure profiles allows access to higher normalized beta and lower internal inductance. Second, the correction of a poloidal field coil induced error-field has largely eliminated locked tearing modes during normal operation and increased the maximum achievable beta. As a result of these improvements, peak beta values have reached (not simultaneously) beta-t = 35%, beta-N = 6.5, volume-average beta-N = 4.5, beta-N / li = 10, and beta-P = 1.4. High beta-P operation with reduced tearing activity has allowed a doubling of discharge pulse-length to just over 1 second with sustained periods of beta-N = 6. Details of the beta limit scalings and beta-limiting instabilities in various operating regimes are described.

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IAEA 2003